This invention relates generally to apparatus for the automatic or automated casting of articles. The invention can be used in casting or pouring a wide variety of items both as to type and as to material. An exemplary application of the invention is in the casting of aluminum pistons for internal combustion engines. A primary object of the invention is to provide reliable automated pouring ladle equipment having improved simplified construction and operation over prior pouring approaches. Another object of the invention is to provide the necessary interfacing or interlocking control means for use with known automated equipment such as metal holding furnaces and automatic molding machines. It is also an object of this invention to provide automated pouring equipment which is compatible with current space requirements for production piston molding facilities.
Most of the pistons cast in the automotive industry today are cast manually. Certain problems exist in the manual pouring of pistons which make automated pouring means desirable. The quality of permanent mold pistons produced by manual pouring is controlled to a major extent by human elements. This fact is obvious to those familiar with this art since they know that there are differences in quality and productivity between individual pourers even though extensive mechanical factors have been provided to assist the pourer in his task.
In general, various expedients have been used at one time or another in an attempt to obtain low scrap and high production rates, such as temperature and metallurgical control of the metal; closely timed cooling cycle of the poured metal; closely controlled mold dimensions; temperature and flow rate control of the mold cooling system and the cooling water, and scale formation prevention in the water cooling system for the mold. All of these expedients are based on the belief that metal of the same composition, poured at the same temperature, and in the same manner, into molds of the same shape and under the same cooling conditions, will produce the same finished product each time. Furthermore, if these conditions are correctly set, it is believed that the quality of the product will be uniformly high and more satisfactory for use.
In spite of all of the controls of the type listed above which have been used, scrap still occurs erratically and on a continuing basis. The majority of this scrap can be traced to one factor which is completely variable, that is the man pouring the metal. Well trained and conscientious operators can produce a good product with very little scrap attributable to pouring per se. However, the job is hot and arduous and a relatively high labor turn-over exists. New men in training and other human factors tend to produce scrap and low productivity.
The major operator attributable scrap causes are: mis-runs, flash, trapped gas and inclusions.
Mis-runs are caused by metal not running into areas of the mold which it should normally fill. This can occur because metal traveling to the area in question freezes before that particular mold cavity area can be completely filled with molten metal.
Three main human factors appear to be active in this regard. The transfer of metal from the furnace to the mold slowly causes the operator to start his pour with colder metal than a faster transferring operator. Pouring the transferred metal at slow rates delivers cooler metal into the mold cavity areas than pouring at faster rates. Also pouring fewer pistons per hour causes operation with cooler molds than producing at a faster rate.
Flash is the reverse of mis-runs. It is caused by metal running into mold joints where it is not desired and where it forms thin pieces of excess metal. It is less common as an operator fault than mis-runs since the main human factor which produce it are less frequently encountered.
Again, three primary human factors appear to be operative in causing flash. Fast transfer from the furnace to the mold results in hotter metal than is necessary for a good cast article. Faster pouring rates provide hotter metal with greater kinetic energy in the mold joint area. Also, faster production rates result in hotter molds.
Entrapped gas results when metal is poured into a mold in an erratic or turbulent manner so that air is trapped in the entering stream of molten metal causing the formation of large voids in the casting. The gas entrapped in the molten metal stream does not break out from the molten metal at a mold/metal surface for two main reasons. Firstly, the inside of the void becomes oxidized as soon as the air is entrapped and metal outside acts as an envelope to contain the gas. Secondly, the solidification is very rapid in the mold and the metal freezes before the entrapped gas can break out of the surface.
Inclusions of metal oxide, refractory particles and the like usually occur when a operator does not skim back the surface of the metal in the furnace before dipping to fill the ladle or when he uses a dirty ladle which contains metal oxide in the form of a skin which lines the ladle from the last pour.
From the above it can be seen that a mechanical, preferably automatic, means for transferring metal from a molten metal holding furnace or the like to a mold should be expected to provide a considerable increase in quality and productivity. However, such a means must overcome the problems associated with molten metal as discussed hereinabove. Attempts to mechanize the pouring process have been previously made. However, at present major permanent mold piston producers continue to hand pour the metal into the molds when manufacturing pistons.
Experience indicates that the following requirements are absolutely necessary for a successful mechanical pouring apparatus. The machine must be rugged. Smooth action is imperative. The action must be fast but well controlled. There should be buffering at the end of machine movements, i.e., there should be no abrupt starting or stopping which tends to wash metal about in the ladle or spill it. The action must be positive in that its movements are reproducible and are all carried out in the same place each time they are repeated.
The following observations have been made in connection with the techniques used by the best hand pourers of pistons. These pourers do not rotate the ladle about its approximate center of gravity but rotate it about a horizontal axis across or normal to the tip of the ladle pouring spout. To start the pour the spout is placed as close into the sprue opening of the mold as possible to minimize the free fall into the mold and the turbulence generated during the pour. A good operator pours rapidly at the immediate start of the pour then appears to reduce the rate of metal delivery somewhat until close to the end of the pour when metal appears at the base of the riser. At this point he tends to raise the ladle somewhat and increase the rate of pour until the riser is full when he cuts off the flow of metal completely.
It is speculated that the fast start with a minimum free fall is necessary to establish fast filling of the lower parts of the mold at a time when these portions of the mold are at their coldest in the casting cycle. Fast pouring at this stage ensures that metal reaches the remote cold parts of the mold. Once flow has been established it is desirable to slow down the rate of pour so that metal fills the mold layer upon layer with the hottest metal always being the highest layer in the mold. Thus, feeding the cooler areas of the mold, which are solidifying and contracting, occurs the hot metal flowing down from the higher areas. The end of this stage of the pouring operation occurs when the cavity has been filled completely and the next task is to fill the riser of the mold. The function of the riser is to act as a reservoir of molten metal to be drawn into the cooler areas of the mold as the metal solidifies and to compensate for the solidification shrinkage which takes place. It is thus an ideal situation that the riser be filled with the hottest metal possible. By increasing the rate of flow from the ladle and sometimes even raising the spout slightly the metal runs into the sprue more quickly and with a higher kinetic energy. This combination ensures that the last metal poured moves rapidly through the sprue and into the riser to give the hottest possible metal in the riser area.